An impact operated ground piercing tool of the general type to which this invention relates is disclosed in U.S. Pat. No. 3,756,328 to Sudnishnikov et al. Such a tool comprises, in general, a sturdy elongated housing, a piston-like striker reciprocable back and forth in the housing, and an air valve coaxially supported in the rear end portion of the housing and to the rear of which a compressed air hose is connectable. The rear portion of the striker is formed as a tubular wall that defines a coaxial rearwardly opening well in which a sleeve formed by an enlarged diameter front end portion of the air valve is received with a close sliding fit, so that the sleeve and the rear portion of the striker cooperate to define a variable volume inner chamber.
Pressure air from the hose, conducted into the inner chamber by way of the air valve, drives the striker forward into percussive engagement against a rearwardly facing anvil surface in the front end portion of the housing, and the momentum produced by this impact drives the housing forward in the ground.
As the striker nears the front of its stroke, radially opening ports through its tubular wall portion pass the front edge of the sleeve, and are thus uncovered, so that pressure air can flow out of the inner chamber and into an outer chamber jointly defined by the inside of the housing and the outside of the striker. Pressure air in this outer chamber acts upon the entire cross-sectional area of the striker to exert a force that propels the striker rearward until the ports in it are behind the sleeve, whereupon pressure air from the outer chamber is exhausted to the atmosphere through the ports in the striker by way of rearwardly opening outlet passages in the housing that are external to the sleeve.
The air valve must be concentrically supported in the rear portion of the housing, to be in concentric relation to the reciprocating striker. Because of the high acceleration of the housing at each advance and the inertia forces that tend to retard the air valve and the hose that is attached to it, high forces have to be transmitted through the connection between the housing and the air valve, and therefore a rigid connection between them, unless it is adequately strong and heavy, can break after a relatively short period of operation. Thus a yielding shock dampening connection between the housing and the air valve in conjunction with a certain amount of flexibility in the sleeve of the air valve is more likely to have a long useful life than a rigid connection.
It is evident that the junctures between the rear portion of the striker and the inner wall of the housing and between the sleeve and the inner wall of the rearwardly opening well of the striker must be simultaneously freely-slidable and well-sealed against air leakage through the juncture. In practice, obtaining optimal sliding freedom and sealing has been extremely difficult. This is especially so in tools which utilize only metal-to-metal contact at these crucial junctures.
Typical of such metal-to-metal tools is the tool described in Sudnishnikov U.S. Pat. No. 3,756,328 referred to earlier. In such tools, adequate freedom to slide at the crucial junctures can only be accomplished by adopting a clearance fit between the parts at the juncture. Typically, those clearances are on the order of 0.015 of an inch. If the clearances are increased, air sealing and tool efficiency suffer. If the clearances are decreased, sliding friction goes up and tool efficiency suffers.
The need for carefully controlled metal-to-metal fit at the crucial junctures in prior art tools increases demand on manufacturing quality control and assembly. Further, because ground borers operate in a very wide range of hostile environments and temperatures and with varying and unpredictable degrees of field maintenance, clearances at the crucial junctures can vary unpredictably in the field. Further, in operation, the tools often strike rocks, tree roots and other obstructions, thereby, distorting the housing of the borer and slowing down or possibly jamming the striker against the close-fitting inner wall of the housing or against the sleeve of the air valve. The result is erratic tool operation and earlier than expected tool failure. Further, if the tool jams in operation, it often must be excavated out or abandoned at a potential cost of thousands of dollars.
Applicant has discovered that increased tool speed, efficiency, power, and reliability can be achieved at one of the crucial junctures by the use of spot bearings and a sealing bearing of low surface area on the anterior and posterior portions of the striker. These improvements reduce sliding friction between the striker and the housing and heighten air pressure retention in the outer chamber during rearward motion of the striker. While earlier tools such as the Accupunch made by Tracto Technik, Lennestadt, West Germany and the Terra Hammer made by Terra AG, Mullerweg, West Germany have attempted to use antifriction and sealing materials on their strikers, none has done so with the simplicity, efficiency and high reliability afforded by applicant's invention.
Additionally, applicant has discovered that a sizable reduction in sliding friction with enhanced sealing can be achieved at the other crucial juncture, (i.e., between the sleeve and the interior of the striker wall) by constructing the sleeve of the air valve of a flexible plastic material from a group which has good antifriction properties and by use of the bearing land configuration of applicant's design. By use of such a sleeve, useful reduction in sliding friction between the sleeve and the striker inner wall is effected as well as improved air sealing between the striker and the sleeve. Heightened tool efficiency and reliability are thus achieved.
Further, applicant has discovered that additional flexibility in the sleeve itself can provide significant reduction in the effect of shock and flexure on the tool. While certain earlier tools, including the one described in Sudnishnikov, et al. U.S. Pat. No. 3,410,354 and the tool known as the Ground Bullet made by Lewin Engineering Co., Mound, Minn., have utilized a rubber or plastic hose to replace the posterior portion of the sleeve and to connect the sleeve to the air inlet pipe, applicant has discovered that the entire sleeve may advantageously be made of a flexible plastic material. Shock and the effects of side-to-side pressure of the air inlet hose on the air inlet pipe are reduced by such a structure.
Further, applicant has discovered that even greater reliability can be achieved by using a flexible plastic sleeve in conjunction with an elastomeric shock dampener.
Applicant has further discovered that the invention described herein is essentially a necessity when the tools are made in small diameters of less than about 3 inches. In small diameter tools, the amount of room in the posterior end of the tool becomes less and less until the shock dampening coupling becomes so firm that its ability to flex is reduced to the point where it is only marginally adequate to deal with the shock and pressure of tool operation in the field and a flexible sleeve is a practical necessity.